1. Field of the Invention
The present invention relates to a mouth guard composition which is rich in formability, viscoelasticity and impact resistance and suits the purpose of protecting the teeth, jawbones and intra-oral tissues from external forces.
2. Prior Art
Injury accidents occur frequently in sport games such as boxing, rugby or football. Most of the injuries are said to be caused by fracture of the teeth and jawbones. Protecting the interior of the mouth from such external wounds demands a material having suitable strength to external forces and an effect of cushioning external pressures.
For such protective materials (mouth guards), polyvinyl chloride (PVC), especially, ethylene/vinyl acetate copolymers (PVAc-PE) have generally been used.
Since intra-oral geometries differ from one person to another, if the materials well fitted the intra-mouth use can be easily made, such materials could be better for a mouth guard. Thus, the mouth guards materials should have the properties of imparting suitable elasticity to mouth guards, being well-fitted to intra-oral geometries and providing sufficient protection against external pressures.
Moreover, the mouth guard materials have to be supplied in such forms that allow dental clinicians to readily make mouth guards to the order of individuals, taking into account their cleanliness, taste, durability, the ease with which words are pronounced, etc.
The conditions which are required of materials used to protect the teeth, jawbones, etc. are that they should resist to occulsal pressures (withstand tearing-off by biting), be easy to wear, cushion external pressures to provide sufficient protection, be securely supplied, be easy to make and be of durability. Of the physical properties meeting such necessary conditions, the elasticity, tensile strength and elongation relate to the relaxation of external pressures and the protection and durability of the teeth and jawbones and, further, they represent the degree of the difficulty with which mouth guards are worn and maintained in place. The degree of water absorption stands for the degree of stability of the materials.
Generally available products have a hardness of 50-90, a tensile strength of 30-200 kg/cm.sup.2, an elongation of 300-900% and a water absorption of 0.5-2.5%.
For mouth guards of such quality, polyvinyl chloride (PVC) or ethylene/vinyl acetate copolymers (PVAc-PE) have been used as their component.
PVC (polyvinyl chloride) is prone to be degraded by heat, light, etc. and not sufficiently durable. For that reason, it usually contains a stabilizer, and is additionally mixed with a plasticizer to adjust its softening temperature. However, such regulators are often undesirable to the human body because of their leaching.
PVAc-PE (ethylene/vinyl acetate copolymers) whose hardness and elasticity are regulated by the degree of polymerization and the ratio of polyethylene to polyvinyl acetate, are relatively often used as mouth guards. However, PVAc-PE can only be softened and formed at a temperature of 80.degree.-100.degree. C. by limiting both the degree of polymerization and the ratio of polyvinyl acetate to polyethylene to specific ranges so as to maintain the properties to resist impacts and protect teeth and jawbones and fit teeth. Another grave problems with PVAc-PE are that they are poor in durability and so poor in strength that they are easily torn off, they become too hot because of their high softening manipulation temperature, and are prone to be sticky to the hand at the time of forming.